A light strikes the boundary of a medium at a 45 degree angle instead of entering the second medium the light reflects back into the first medium what can you conclude about the indexof refraction of the second medium

The phenomenon that works here is called Total internal reflection

Explanation:

When the angle of incidence is increased past the critical angle, the refracted angle will not exit the medium and all incident light will reflect back off a boundary back into the first medium.

This phenomenon is called Total Internal Reflection. Total Internal Reflection will only occur with a large angle of incidence that is greater than the critical angle.

Now, when we talk about the second medium  

using the Law of refraction

Sini/Sinr =refractive index of medium 2/ refractive index of medium1

when the light ray passes from denser medium to rarer medium TIR happens along with angle of incident should be greater than critical angle. In this case incident angle is 45 degrees

conclusion: medium 2 is rarer here than medium 1